It is known that roasted coffee when packed in commercial packaging will generally retain its fresh aroma for only about 8 to 10 weeks. Then an aging process sets in, oxygen-catalyzed condensation and polymerization reactions taking place, possibly with the formation of peroxides in small quantities which impart a sensorially undesired note to the coffee aroma and taste.
Attempts have therefore been made in various ways to develop coffee packaging in which there remains no more than a minimum of oxygen, such packaging forms being, for example, hermetic vacuum hard packs, hermetic soft packs evacuated and subsequently filled with a shielding gas, vacuum cans, etc. While in the case of ground coffee improvements were achieved thereby, with coffee in whole bean form the problem of slow CO.sub.2 regassing from the beans occurred. The reason is this: In the roasting process, besides the formation of the brown color and of the coffee aroma, much CO.sub.2 is released, which for the most part is included in the roasted beans. This gas, which amounts to a multiple of the bean volume (Lit.: R. Radtke et al: Kaffee & Tea Markt 25 (17), 7-14 (1975)), diffuses out of whole beans slowly in preponderant degree during the first two or three weeks and causes an undesirable bloating of the hermetic packing. With ground coffee, this effect is practically no longer observable, as the CO.sub.2 is much more rapidly released from the roasted coffee during the grinding process.
The attempt has been made, therefore, to solve the problem of the gradual CO.sub.2 desorption in roasted beans by using vacuum cans which are designed to withstand an increased internal pressure, or by welding CO.sub.2 -adsorbing substances, packed in small polyethylene sacs, into the laminate foil which serves as packaging material, or by using a mechanical valve which opens at a certain CO.sub.2 pressure and can be welded into a gasproof package in known manner.
The operational safety of such pressure relief valves has been improved before by using in support of the valve effect, a liquid layer of high cohesive force as has been known for a long time, in greased ground valves. In an arrangement known from German laid-open application OS No. 2,360,126 (British patent specification No. 1,434,660 and U.S. Pat. No. 3,799,427 being counterparts thereof), a rubber disk serving as valve element lies on a valve seat which, like the rubber disk, is coated with a silicone oil film. The disk type valve body can lift off its valve seat only when the internal pressure present in the package has overcome the sum of the elastic reaction of the valve body and the adhesive force of the viscous intermediate layer between the valve body and valve seat. As small pressure forces are not sufficient to release the adhesive force of the viscous intermediate layer, the pressure relief valve opens only at certain excess pressure, so that the deflection of the valve element is relatively great and therefore a certain period of time passes before upon cessation of the excess pressure, the valve element has again approached the valve seat to the extent that, due to the viscous intermediate layers, joint action of the adhesive forces occurs again and thus complete tightness exists again, preventing any undesired access of gas, e.g., air.
Prior art pertinent to the present invention in addition to the above-mentioned German OS, includes U.S. Pat. Nos. 2,361,344; 2,870,954; 3,088,255; 3,672,915; 3,799,427 and 4,000,846, as well as British patent No. 1,169,280.